Been following this thread with interest so thought I might as well throw my .02 cents in. Toured for several years on a Jamis Nova with Avid canti's. Got the job done, but noisy and really not that stellar performance, although more than capable of locking up the wheels. Now have a disc equiped Marinoni Turismo. The major differences that I have noticed is far superior modulation with discs along with consistant performance regardless of weather conditions. The Marinoni actually feels like it brakes and stops better than my Specialized Tarmac.However, this is is a subjective feeling and I have no empericaldata to back up this feeling. As I have never had mechanical problems with either system I can't comment on the ease of repairs or parts availability.
To put everything into perspective, I live in a fairly mountainous region where descents of 10 to 20 Kms (6 to 12 miles ) with grades from 6 to 10 percent are quite common.. My feelings are discs are the wave of the future and I'm currently looking for a disc equiped road bike.
Cheers